In associating a parison or preform which is formed of a thermoplastic material and is at ambient temperatures, and is often of generally tapered or conical configuration, with a complementally shaped heated mandrel, one of the important considerations is to assure uniform and firm surface engagement between the preform and the surface of the mandrel with resultant good thermal contact therebetween and to maintain that thermal contact until the preform reaches molding temperatures. This is usually accomplished by vacuum holding of the preform to the mandrel which is heated to impart molding temperature to the preform in preparation for relative positioning of the associated mandrel and preform to a female mold for re-shaping the preform to the desired configuration. This is usually accomplished by applied differential pressure such as vacuum forming or blow molding. To produce a satisfactory re-shaped product, that is, one without undesirable variations in shape and thickness, the preform should be maintained in uniformly firm engagement with the mandrel surface during heating. Prior attempts have been made in this direction by sand blasting or otherwise roughening the mandrel surface for good vacuum withdrawal of air or by similarly or equivalently configuring the inner surface of the preform as it is pre-molded, and which configured inner surface portions of the preform will effectively disappear as the preform is softened under applied heat. A difficulty has been encountered in that preforms made of most known thermoplastic materials tend to initially expand and thereafter contract along the mandrel as they are heated from ambient to molding temperatures. While a substantial air seal must be maintained between the upper rim portion of the preform and the mandrel assembly, if the upper end portion is restrained against creeping along the mandrel, as by a tight surface fit or abutment with a portion of the mandrel assembly, initial heating and expansion may result in a loosening or separation of lower portions of the preform from the mandrel surface with consequent loss of good thermal contact therebetween and resultant undesirable variations in shape and thickness which may show up in the re-shaped configuration imparted by the female mold under applied differential pressure upon cessation of the applied original vacuum. Further, if the area of the preform surrounding the vacuum draw passageways softens agaisnt those passageways before complete air evacuation occurs throughout the major body areas of the preform, localized air entrapment may occur with the result that those areas will be improperly heated.